The foot is the foundation on which the human body interacts with, and distributes, forces from the ground. It is a complex system made up of twenty-six bones, thirty-three joints and hundreds of muscles, tendons and ligaments. To function properly, all of these complex systems must work together, and a malfunction in any part of the foot may also manifest itself in other locations in the body.
Structurally, the foot has three main parts: the rearfoot, the midfoot and the forefoot. With reference now to FIGS. 1 to 4, an exemplary skeletal foot is shown generally at 10, and comprises rearfoot 12, midfoot 14, and forefoot 16. The skeletal structure of the rearfoot 12 comprises two bones, namely the talus 18 and the calcaneous 20, as well as three joints. These three joints are the talocrural joint 22, between the upper portion of the talus 18 and the lower tibia 24, the subtalar joint 26, between the lower portion of the talus 18 and the upper portion of the calcaneous 20, and the midtarsal joint, which, although conventionally referred to as a single joint, comprises the medial joints 28A between the talus 18 and the navicular bone 32, and the lateral joint 28B between the calcaneous 20 and the navicular bone 32. The midfoot 14 comprises five bones, namely the navicular bone 32, the cuboid bone 34 and three cuneiform bones 36, and forms the arch 38 of the foot 10. The forefoot 16 comprises nineteen bones, namely five metatarsal bones 40 and fourteen phalange bones 42. The fibular malleolus (also referred to as the lateral malleolus because of its position on the lateral side of the foot), which is a protrusion at the lower end of the fibula, is indicated with the reference numeral 46. Similarly, the tibial malleolus (also referred to as the medial malleolus because of its position on the medial side of the foot) is a protrusion at the lower end of the tibia and is indicated with the reference numeral 48.
The movements of the foot 10 relative to the ankle are primarily controlled by way of the rearfoot 10. The primary movements of the talocrural joint 22 are plantar flexion (downward movement of the foot 10) and dorsiflexion (upward movement of the foot 10). These movements take place in the sagittal plane and are important for the movement of the foot and ankle. The movements supported by the subtalar joint 26 are complex; the primary movements enabled by the subtalar joint 26 are inversion (turning the bottom or sole of the foot 10 inward) and eversion (turning the bottom or sole of the foot outward). The subtalar joint 26 creates movements in all three cardinal planes and functions like a mitred hinge. with simultaneous motion in all three cardinal planes. As the subtalar joint 26 moves into eversion, the tibia 24 rotates.
These four movements, plantar flexion, dorsiflexion, inversion and eversion, along with adduction (twisting the foot 10 inward) and abduction (twisting the foot 10 outward) are involved with two main movements during the gait cycle (i.e. walking or running), namely pronation and supination. Pronation is the combination of dorsiflexion, abduction and eversion. Supination is the combination of plantar flexion, adduction and inversion. Supination and pronation are commonly used to define foot alignment under a weight bearing (“closed-chain”) condition. These motions are important to proper movement of the foot 10 throughout the entire gait cycle.
The gait cycle can be divided into three main components. The first is heel strike, which is the period of time in which the calcaneous 20 (the heel) strikes the ground (or other surface) and the rest of the foot 10, i.e. the midfoot 14 and forefoot 16, moves toward the ground. The second stage is midstance, which is the point at which the bottom of the entire foot 10, that is, rearfoot 12, midfoot 14 and forefoot 16, is on the ground. The third stage is toe-off, which is the point at which the foot 10 is preparing to leave the ground, and only the bottom of the forefoot 16 remains in contact with the ground.
During heel strike the foot 10 is in a supinated position, which allows the twenty-six bones to be “locked” so that the foot 10 will have a stable base of support as the calcaneous 20 hits the ground. As the gait cycle progresses to midstance, the foot 10 transitions from the “locked”, supinated position to an “unlocked” pronated position, to allow the foot 10 to accommodate uneven surfaces. As the foot 10 transitions to toe-off, the foot 10 returns to the supinated position to give the foot 10 a stable base of support to push off the ground. Throughout the gait cycle, the foot 10 is in constant motion allowing for both stability and flexibility.
The subtalar joint 26 (see FIG. 1) plays an important role in the foot 10, both statically and dynamically, because of its central role in pronation and supination, and also because it can convert foot rotation into leg rotation and leg rotation into foot rotation. The subtalar joint 26 therefore has a direct role not only in the function of the foot 10, but also the knees, the hips and the pelvis, and an indirect role in respect of areas above the pelvis including the lower back.